The present invention relates to micro electro-mechanical (MEM) tunneling sensors using dual wafers which are bonded together preferably eutectically.
The present invention provides a new process of fabricating a single crystal silicon MEM tunneling devices using low-cost bulk micromachining techniques while providing the advantages of surface micromachining. The prior art, in terms of surface micromachining, uses e-beam evaporated metal that is patterned on a silicon dioxide (SiO2) layer to form the control, self-test, and tip electrodes of a tunneling MEM sensor. A cantilevered beam is then formed over the electrodes using a sacrificial resist layer, a plating seed layer, a resist mold, and metal electroplating. Finally, the sacrificial layer is removed using a series of chemical etchants. The prior art for bulk micromachining has utilized either mechanical pins and/or epoxy for the assembly of multi-Si wafer stacks, a multi-Si wafer stack using metal-to-metal bonding and an active sandwiched membrane of silicon nitride and metal, or a dissolved wafer process on quartz substrates (Si-on-quartz) using anodic bonding. None of these bulk micromachining processes allow one to fabricate a single crystal Si cantilever (with no deposited layers over broad areas on the beam which can produce thermally mismatched expansion coefficients) above a set of tunneling electrodes on a Si substrate and also electrically connect the cantilever to pads located on the substrate. The fabrication techniques described herein provide these capabilities in addition to providing a low temperature process so that CMOS circuitry can be fabricated in the Si substrate before the MEMS sensors are added. Finally, the use of single crystal Si for the cantilever provides for improved process reproductibility for controlling the stress and device geometry.
Tunneling sensors may be used in various military, navigation, automotive, and space applications. Space applications include satellite stabilization in which MEM sensor technology can significantly reduce the cost, power, and weight of the presently used gyro systems. Automotive air bag deployment, ride control, and anti-lock brake systems provide other applications for MEM sensors. Military applications include high dynamic range accelerometers and low drift gyros.
Generally speaking, the present invention provides a method of making a micro electro-mechanical sensor wherein a cantilevered beam structure and a mating structure are defined on a first substrate or wafer and at least one contact structure and a mating structure are defined on a second substrate or wafer. The mating structure on the second substrate or wafer is of a complementary shape to the mating structures on the first substrate or wafer. A bonding or eutectic layer is provided on at least one of the mating structures and the mating structure are moved into a confronting relationship with each other. Pressure is then applied between the two substrates and heat may also be applied so as to cause a bond to occur between the two mating structures at the bonding or eutectic layer. Then the first substrate or wafer is removed to free the cantilevered beam structure for movement relative to the second substrate or wafer. The bonding or eutectic layer also provides a convenient electrical path to the cantilevered beam for making a circuit with the contact formed on the cantilevered beam.
In another aspect, the present invention provides an assembly or assemblies for making a single crystal silicon MEM sensor therefrom. A first substrate or wafer is provided upon which is defined a beam structure and a mating structure. A second substrate or wafer is provided upon which is defined at least one contact structure and a mating structure, the mating structure on the second substrate or wafer being of a complementary shape to the mating structure on the first substrate or wafer. A pressure and heat sensitive bonding layer is disposed on at least one of the mating structures for bonding the mating structure defined on the first substrate or wafer with the mating structure on the second substrate in response to the application of pressure and heat therebetween.